The invention relates to a new and useful power recovery system to reduce the energy requirements of many industrial and liquid purification processes that involve pumping liquid or gases at high pressures. This invention is particularly well-suited for use in reverse osmosis processes which are used to remove salt from sea water. In the reverse osmosis system and other systems in which the invention is particularly well-suited for use, a liquid or gas is pumped at high pressure into a chamber. In the chamber a portion of the liquid or gas is purified or otherwise processed and drawn from the chamber. The remainder of the high pressure gas or liquid is discharged from the chamber as reject that is disposed. The reject is usually at a very high pressure and this pressure must be dissipated through the use of a throttling valve or other device. Usually the throttling valve reduces the pressure in the reject stream to essentially 0 psi so that all of the pressure energy in the reject stream is dissipated and provides no further benefit to the process. These energy losses can be very significant and result in a basic inefficiency in the system. In the use of reverse osmosis methods to remove salt from sea water these high energy costs associated with such an inefficient system have severely limited the commercial applications of the technology.
Several methods have been tried to recover the energy in the reject stream in order to reduce the operating costs of industrial or liquid purification processes. With regard to reverse osmosis systems, pistons with mechanically actuated valves that are positioned in devices resembling steam piston engines have been used. However, these devices have found no commercial acceptance due to the high cost and high maintenance required due to the mechanically complex designs of the system. Furthermore, these systems produce strong shockwaves in the feed flow called water hammer, each time a valve opens or closed resulting in possible damage to components of the reverse osmosis system.
Other systems have used a turbine driven by the high pressure reject that is discharged from the system. The turbine is connected to the motor operating the feed pump. For good efficiency, a turbine must operate at very high speeds, usually exceeding 15,000 rpm. This high speed means that a reducing gear box must be installed between the turbine unit and the feed pump motor to effectively transfer the power from the turbine to the feed pump motor. The reducing gear box is a very expensive piece of equipment and requires a great deal of skill to install and maintain properly. A gear box also requires external means of lubrication further increasing the maintenance costs. It is also possible that the lubrication can contaminate the water that is feed to the reverse osmosis system. High speed seals must also be provided on the shaft between the turbine and the speed reducing gear box. These high speed seals are also expensive and usually not very reliable in field applications. For the above reasons there has been very limited commercial acceptance of this type of power recovery system for use in reverse osmosis processes.
Other methods include the power recovery pump turbine systems which are described in U.S. Pat. Nos. 4,983,305; 4,966,708 and 5,048,045, in which were so invented by the inventor herein and are expressly incorporated by reference here.